Carbon nanotube manufacturing method

ABSTRACT

In a carbon nanotube manufacturing method, a substrate made of a carbon-containing metal material is arranged in a reactor in which a carbon source gas has been introduced. A plurality of carbon nanotubes are grown at a first temperature by chemical vapor deposition. Thereafter, the substrate is heated at a second temperature lower than the first heating temperature to grow the plurality of carbon nanotubes longer on the substrate.

BACKGROUND OF THE INVENTION

The present invention relates to a carbon nanotube manufacturing method of forming a plurality of carbon nanotubes on the surface of a substrate.

A carbon nanotube forms a completely graphitized cylinder having a diameter of about 40 nm to 50 nm and a length of about 1 μm to 10 μm. Examples of the carbon nanotube include one having a shape in which a single graphite layer (graphene) is closed cylindrically and one having a shape in which a plurality of graphenes are layered telescopically such that the respective graphenes are closed cylindrically to form a coaxial multilayered structure.

The central portions of the cylindrical graphenes are hollow. The distal end portions of the graphenes may be closed, or broken and accordingly open.

It is expected that the carbon nanotube having such a special shape may be applied to novel electronic materials and nanotechnology by utilizing its specific electronic physical properties. For example, the carbon nanotube can be used as an electronic emitting source for an electron tube. When a strong electric field is applied to the surface of a solid, the potential barrier of the surface of the solid which confines electrons in the solid becomes low and thin. Consequently, the confined electrons are emitted outside the solid due to the tunnel effect. These phenomena are called field emission.

In order to observe field emission, an electric field of as strong as 10⁷ V/cm must be applied to the solid surface. To realize this, according to one scheme, a metal needle with a sharp point is used. When an electric field is applied by using such a needle, the electric field is focused on the sharp point, and a necessary strong electric field is obtained.

The carbon nanotube described above has a very sharp point with a radius of curvature on the nm order, and is chemically stable and mechanically tough, thus providing physical properties suitable for the material of a field-emission emitter.

When the carbon nanotube having the characteristic feature as described above is to be used for, e.g., an electronic emitting source in an electron tube such as an FED (Field Emission Display), carbon nanotubes must be formed on a substrate having a large area.

Carbon nanotube manufacturing methods include electric discharge in which two carbon electrodes are separated from each other by about 1 mm to 2 mm in helium gas and DC arc discharge is caused, laser vapor deposition, and the like.

With these manufacturing methods, however, the diameter and length of the carbon nanotube are difficult to adjust, and the yield of the carbon nanotube as the target cannot be increased very much. A large amount of amorphous carbon products other than carbon nanotubes are produced simultaneously. Thus, a refining process is required, making the manufacture cumbersome.

In order to solve these problems, a method of preparing a catalyst metal layer on a substrate, and heating the substrate and supplying a carbon source gas onto the catalyst metal layer, thus growing a large amount of carbon nanotubes from the catalyst metal layer by chemical vapor deposition (CVD) is proposed (see Japanese Patent Laid-Open No. 2001-048512). With the carbon nanotube manufacture in accordance with chemical vapor deposition (CVD), the length and diameter of the carbon nanotube to be formed can be controlled in accordance with the type of the catalyst metal, the duration of growth, and the type of the substrate.

When the carbon nanotube is used as the electronic emitting source, if a thinner carbon nanotube is used, electrons can be emitted with a lower voltage. For example, when the carbon nanotube is used as an electronic emitting source for an FED, if a thinner carbon nanotube is used, driving with a lower voltage is enabled. This is preferable in terms of power consumption saving.

When the carbon nanotube is formed by chemical vapor deposition (CVD), a plurality of carbon nanotubes can be formed close to each other on a substrate. When the substrate temperature is set as high as 800° C. to 1,000° C., a thin carbon nanotube having a diameter of about 10 nm can be formed.

When the carbon nanotube is grown at a high temperature, a thin carbon nanotube with a diameter of about 10 nm can be obtained. This is suitable as an electronic emitting source for low-voltage driving. However, the growing speed of the carbon nanotube per unit time is low. To obtain a carbon nanotube having a desired length, much time is needed.

When the carbon nanotube is grown at a high temperature, part of a layer including a plurality of carbon nanotubes formed on the substrate may separate from the substrate, or cracking may occur, to form steps on the surface of the layer. Therefore, it is difficult to form a carbon nanotube layer uniformly.

In this manner, when the height of the layer including the carbon nanotubes formed on the substrate varies locally, local field concentration occurs on the highest (longest) carbon nanotube, thus causing field emission locally. Local field emission leads to destruction of the carbon nanotubes. Depending on the case, a chain of destruction of a large number of carbon nanotubes is caused. When destruction of the carbon nanotubes serving as an electronic emitting source occurs, stable field emission cannot be obtained.

SUMMARY OF THE INVENTION

It is, therefore, the principal object of the present invention to provide a method of manufacturing a carbon nanotube which is thinner than in the prior art and with which a uniform electronic emitting source can be obtained.

It is another object of the present invention to provide a carbon nanotube manufacturing method with which a plurality of carbon nanotubes which are thinner than in the prior art can be manufactured quickly.

It is further object of the present invention to provide a method of manufacturing a carbon nanotube which can form a layer including a plurality of thinner carbon nanotubes uniformly on a substrate.

In order to achieve the above objects, according to the present invention, there is provided a carbon nanotube manufacturing method comprising arranging a substrate made of a carbon-containing metal material in a reactor in which a carbon source gas has been introduced, and growing a plurality of carbon nanotubes at a first temperature by chemical vapor deposition, and thereafter heating the substrate at a second temperature lower than the first heating temperature to grow the plurality of carbon nanotubes longer on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are sectional views showing the steps in a carbon nanotube manufacturing method according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment of the present invention will be described with reference to the accompanying drawings.

FIGS. 1A to 1C show a carbon nanotube manufacturing method according to an embodiment of the present invention. First, as shown in FIG. 1A, a substrate 101 made of stainless steel such as a 426-alloy is prepared.

Subsequently, as shown in FIG. 1B, the substrate 101 is placed in a reactor 104 formed of, e.g., a quartz pipe. While a carbon source gas and hydrogen gas (carrier gas) are supplied from one side of the reactor 104, the substrate 101 is heated by a heater 105. FIGS. 1B and 1C schematically show the section of the reactor 104.

In the chemical vapor deposition growth process using the reactor 104 described above, carbon monoxide gas may be used as the carbon source gas, and its flow rate may be set to about 500 sccm. The flow rate of the carrier gas may be set to 1,000 sccm. As the carbon source gas, one of C1 to C3 hydrocarbon gases such as acetylene, ethylene, ethane, propylene, propane, and methane gases can be used. In the above process, as the substrate 101, one made of stainless steel is used. However, the present invention is not limited to this. It suffices as far as the surface of the substrate where carbon nanotubes are to be formed is made of a material containing a metal from which carbon nanotubes grow by chemical vapor deposition. The metal is any one of, e.g., iron, nickel, cobalt, and chromium, or an alloy of them.

According to this embodiment, the heating temperature for the substrate 101 is set as high as at about 800° C. to 900° C., and chemical vapor deposition is performed for 10 min. Then, as shown in FIG. 1B, a plurality of carbon nanotubes 102 each having a diameter of about 10 nm grow on the surface of the substrate 101. The carbon nanotubes 102 grow to have a length of about 1 μm. In this case, for example, the plurality of carbon nanotubes 102 extend upright closely on the surface of the substrate 101.

After the above chemical vapor deposition is performed, subsequently, the heating temperature of the heater 105 is decreased to heat the substrate 101 at a low temperature of about 650° C. Chemical vapor deposition is performed for 20 min.

Then, the carbon nanotubes 102 that have grown on the surface of the substrate 101 grow longer. As shown in FIG. 1C, a plurality of carbon nanotubes 103 are formed uniformly to have a length of about 13 μm. As a result, a carbon nanotube layer having a uniform thickness is formed on the substrate 101. In the carbon nanotube layer, for example, a plurality of carbon nanotube fibers are entangled with each other to be fluffy. The high-temperature treatment may be performed within the range of 750° C. to 1,000° C. If the temperature is less than the lower limit of the above range, CNTs cannot be formed; if the temperature exceeds 1,000° C., it is not preferable because problems may occur in heat resistance of the substrate and quartz pipe. The low-temperature treatment as the second step may be performed within the range of 500° C. to 750° C. When the temperature was lower than 750° C., preferably near 650° C., CNTs can grow from the metal substrate quickly.

As has been described above, according to the present invention, the substrate is heated at the first temperature, and a plurality of carbon nanotubes are grown on the surface of the substrate by chemical vapor deposition. Subsequently, the substrate is heated at the second temperature lower than the first temperature, so that the plurality of carbon nanotubes grow longer. As a result, according to the present invention, the thin, about 10-nm diameter carbon nanotubes which have grown in the early stage grow longer in the second stage with a faster growing speed after the temperature is decreased. Therefore, a layer including a plurality of carbon nanotubes which are thinner than in the prior art can be uniformly formed on the substrate. 

1. A carbon nanotube manufacturing method of arranging a substrate made of a carbon-containing metal material in a reactor in which a carbon source gas has been introduced, and growing a plurality of carbon nanotubes at a first temperature by chemical vapor deposition, and thereafter heating the substrate at a second temperature lower than the first heating temperature to grow the plurality of carbon nanotubes longer on the substrate.
 2. A method according to claim 1, wherein at least a surface of the substrate is made of a metal material containing any one of iron, nickel, cobalt, and chromium.
 3. A method according to claim 1, wherein the first temperature is 750° C. to 1,000° C., and the second temperature is 500° C. to 750° C.
 4. A method according to claim 1, wherein the carbon source gas is one gas selected from the group consisting of carbon monoxide, acetylene, ethylene, ethane, propylene, propane, and methane gases. 